How to Use a Motorcycle Engine Straight Edge

In this post I discuss how to use a motorcycle engine straight edge (also sometimes referred to as an engineer’s straight edge or machinist’s straight edge). In my last blog post, I described the uses, alternatives, and provided some general advice about this particular tool. In this post, I am really focusing on you use one.

I want to make this a simple 1, 2, 3 style post, so here we go:

How to Use a Motorcycle Engine Straight Edge

Step 1: Gather the tools you need. I bet you can guess what the first one is. That’s right, your straight edge. Also grab, a tiny flashlight, and a feeler gauge.

Step 2: Since a straight edge is a straight line and the surface of interest can be quite a few different shapes, plan out how you are going to determine the overall flatness of the surface. This is done by making a plan of how you are going to lay the straight edge across the surface multiple times such that when you are done, you can confidently say the surface is flat. This is best illustrated with an example. Let’s say, the surface you are checking is the top of a cylinder (i.e. a circle). You want to measure the surface in about 4-8 different directions. Think about how a pizza pie is cut up. The lines created by the pizza cutter divide the circle into delicious slices. This is exactly how you want to check the surface. If you are dealing with a square, start by checking the surface in a box pattern, then check corner to corner, and then maybe a few extra vertical or horizontal cuts.

Step 3: Finally, it is time to execute and check your surface in multiple directions. Lay the straight edge across the surface and take your tiny flashlight. Turn the flashlight on and shine it at the base of the straight edge on the other side. Can you see light near the base of the straight edge on your side? Light travels around corners so if you can see light, there is a gap or unevenness to the surface. Grab your feeler gauge and slowly use the different sizes from smallest to largest to find the gap size. You can skip straight to trying to slip feeler gauges under the straight edge in various spot, but the light trick is a faster “process of elimination.”

I hope it goes without saying that once you have checked one mating surface, that you need to also check the other mating surface. Both obviously need to be within tolerance (as specified in your owner’s manual), otherwise sadly you will be taking the surface to a machine shop to be flattened or “decked.”

Hopefully, this quick little post about how to use a motorcycle engine straight edge has helped those who have ever wondered about the checks required before placing new gaskets.

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